1. Field of the Invention
The present invention relates generally to digital communications. More specifically, the invention relates to a system in which data at a variable rate is transmitted and is received at a communications receiver where the variable rate data is decoded in an efficient, multichannel, multi-rate data decoder.
2. Description of the Prior Art
The most advanced communication technology today makes use of spread spectrum modulation or code divisional multiple access (CDMA) for point-to-multipoint telecommunications. Since the 1950's, CDMA has been used in military applications due to the difficulty in detecting and jamming the communications transmission. This attribute is due to a wireless communication technique that uses a modulated transmission bandwidth much greater than the information bandwidth of the transmitted signal.
A simplified CDMA communication scheme is shown in FIG. 1. A single communication channel of a given bandwidth is mixed with a spreading code. The relatively narrow band modulated signal is spread by sequence to occupy a much wider transmitted bandwidth by multiplication with a unique spreading code. The spreading code comprises a noise-like high-rate pseudorandom sequence or code that becomes part of the transmitted data. The low level noise-like appearance of the resultant transmitted signal is such that it is unlikely to interfere with other spectrum users.
At the receiver, the signal is despread by correlating the received broadband signal with an identical locally generated pseudorandom sequence to resolve the data from a plurality of data signals occupying the same transmission bandwidth. This collapses the signal back to its original bandwidth and also spreads any narrow band radio signals present within the occupied spectrum so that they now appear as noise to the receiver. By using many different pseudorandom code sequences, multiple users may be accommodated within the same transmission spectrum.
The same features that have enabled CDMA communication techniques to succeed in military applications also make CDMA communication systems, particularly Broadband Code Division Multiple Access.TM. or B-CDMA.TM. systems, compelling for efficient use of the crowded commercial radio frequency spectrum. Among the many attributes of the CDMA system is the virtual unlimited capacity of the system. Since each user in a CDMA communication system transmits and receives signals over the same transmission bandwidth, there are less stringent channelization and guardband requirements. Unlike FDMA and TDMA systems where the capacity is limited by the number of discrete channels, the capacity for CDMA systems is limited by interference. Therefore, the number of users able to communicate simultaneously over that given transmission bandwidth is significantly increased.
In addition to voice information, non-voice information alone or a combination of the two may be transmitted to the receiver. Certain communications standards such as the integrated services digital network (ISDN) require a much greater data rate than that of digitized voice. To optimize the communication system, various data rates are transmitted to increase the signal-to-noise ratio (SNR) to all receivers.
One measure of spread-spectrum performance is the system process gain, G.sub.p, which is determined by the ratio of channel bit rate to information bit rate, R.sub.c /R.sub.i. The input and output signal to noise ratios are related as ##EQU1##
It can be seen that the higher the data rate, the more interference is produced and the signal-to-noise ratio will suffer. The reduction of interference directly translates to a capacity increase.
Most CDMA telecommunications systems transmit variable rate data to keep the SNR as great as possible. To achieve this, the transmission data rate is either identified within the system level control message which is part of the signal channel or a given receiver must be able to detect the transmitted data rate.
Since many users share this same spectral transmission channel, interference can be induced from one user to another when there is not enough code isolation between the users. Moreover, the data rate must be known prior to convolutional error correction decoding in either the transmitter or receiver.
Most prior art receivers make use of independent, single-rate convolution decoders to properly reconstruct the digital data once received and despread. Since data rate information for each frame is transmitted, the receiver does not have to determine from the received frame of data the rate at which the data was encoded thereby lessening the complexity of the receiver and increasing overall system speed. However, the use of convolutional decoders dedicated to each transmitted data rate reduces overall processing efficiency and increases system costs.
Accordingly, there exists a need for an efficient, convolutional decoder that can handle variable data rates.